NATURE does not like imbalance, so when there is a DIFFERENCE in the CONCENTRATION of 2 substances - these can be 2 LIQUIDS or 2 GASES, the particles of the one which is most concentrated i.e. the ones which are closer together, begin to spread out - move further away from each other, UNTIL they are EVENLY SPACED OUT - this movement is called DIFFUSION. (See the animation on the right where the purple particles diffuse out of the bottom left-hand corner where they are concentrated until they are all the same distance apart.

Here is an example of SMOKE FUMES (gas) DIFFUSING in the AIR (gases)

Follow this link to see an ANIMATION of HOW DIFFUSION WORKS